The present invention relates to a control system for controlling a driving force distribution between front wheels and rear wheels of a vehicle, and more specifically to a driving force distribution control system capable of preventing hunting during acceleration on a low friction coefficient road surface.
Driving force distribution control systems responsive to a front and rear wheel speed difference are known. A conventional system is designed to combine the handling advantage of the rear wheel drive, and the advantage of the four wheel drive in driving ability by increasing the driving force transmitted to the front wheels continuously with increase in the wheel speed difference. This control system thus controls the driving force distribution so as to reduce the wheel speed difference to zero by feeding back the wheel speed difference.
However, this conventional control system is susceptible to undesirable hunting because of the existence of the feedback loop especially in a starting operation or a medium acceleration on a low friction coefficient road surface. When the front or rear wheels undergo a wheel spin (overspeed rotation) in such a situation, it readily affects the wheel speed difference by affecting the output of the wheel speed sensor, and results in undesirable oscillation of the clutch engagement force of a torque distributing transfer clutch. This oscillation, that is, hunting, causes unpleasant noises and vibrations.
FIG. 13 shows oscillation of the clutch engagement force which is produced by the conventional system when a vehicle having rear wheels directly driven by the engine and front wheels driven through a torque distributing transfer clutch is rapidly started on a slippery road surface. Immediately after the start, as shown in FIG. 13, the rear wheels fall into wheel spin, and causes an increase in the wheel speed difference. In response to this increase in the wheel speed difference, the control system increases the clutch engagement force and increases the engine torque transmitted to the front wheels. By receiving the engine torque, the front wheels increase their speed rapidly because the inertia of the front wheel drive system is smaller than that of the rear drive system, and the front wheels too fall into wheel spin. This front wheel spin causes a sharp decrease in the wheel speed difference, and the control system sharply decreases the clutch engagement force. With the sharp decrease of the clutch engagement force, the front wheels of the smaller inertia regain their gripping ability, and the wheel speed difference is sharply increased again. In this way, the conventional control system causes the clutch engagement force to swing upwardly and downwardly as shown in FIG. 13. During this hunting state, the front wheels of smaller inertia alternate between the state of wheel spin and the state of grip in response to the oscillation of the clutch engagement force, while the rear wheels of greater inertia continue in the wheel spin state.